difference between Linesets and Linestyle?
As I got it, a lineset is a complete "set" of properties, and also has one of the available linestyles, amongst many other indipendent properties and settings. 

You can make each lineset behave in a way that it's applied to different rendering "selection methods".

By "selection methods" you can choose one or more of visibility, edgetypes, facemarks, groups and image border.

Eg: the simplest to explain an understand is perhaps groups: you could have two cubes, each has its own group. Then you can make the cube of Group1 to have a totally different "lineset" than the other, using two of them, each targeted to a particular group, like this:



while the other cube has a different group and, thus, a different lineset, which also has different linestyle (but is not mandatory: it could have same linestyle, but behave differently from the other lineset for other settings...)